With the extensive use of personal computers and other data processing facilities both at home and in the office, a need exists for providing voice and data transmission and switching capabilities on a widespread basis. This has led to the development of the concept of an integrated services digital network (ISDN)--a switched communications network providing end-to-end digital connectivity among network users where voice and data services are provided over the same transmission and switching facilities. Because of the different characteristics of voice and data traffic--voice being typically continuous in one direction for relatively long intervals and tolerant of noise but sensitive to variations in delay, and data being bursty and sensitive to errors but tolerant of moderate delays and delay variations--two fundamentally different switching techniques have been traditionally applied. Circuit switching, where switched connections between users are dedicated for call duration, is the basis of the present-day switched voice telecommunication network. On the other hand, packet switching, where data packets from many calls share a single, high-speed line and are switched based on logical channel numbers included in the packets, was pioneered in the ARPANET network of the U.S. Department of Defense, and has now been implemented in a variety of public data networks.
The approach most commonly used at present to serve customers that have both voice and data requirements is to use separate networks and separate access lines for the two types of traffic. Since both voice and data access lines are typically utilized only a fraction of the time, the duplication of equipment involved in this approach is wasteful and costly compared to an arrangement where facilities could be shared. Some presently available equipment multiplexes both voice and data onto the same access lines. Even if the access lines are shared, however, the voice and data, which are typically transmitted in very different formats, must each be switched to their proper destinations. One approach to the provision of such switching is to use entirely separate packet and circuit switches. This approach, however, is also unnecessarily duplicative. From the line circuits inward, the two information types are served by separate equipment. A primary reason for using this approach is the technical difficulty of handling these two radically different types (and formats) of traffic in an efficient, integrated manner.
A problem arises in an integrated packet switching and circuit switching environment concerning the mechanism to be used for handling the signaling packets that are passed between users and the switching system to establish both circuit-switched and packet-switched calls. A signaling channel must be available to each user on a substantially permanent basis so that calls can be initiated at any time. Routing all signaling packets via circuit switching facilities to a centralized packet switching entity is extremely inefficient particularly in a switching system where the control function is distributed. However, providing individual physical signaling channels from each user directly to the system control is also inefficient.
In view of the foregoing, a recognized problem in the art is the difficulty in providing an ISDN switching system with an efficient mechanism to switch signaling packets from many users to control the establishment of circuit-switched and packet-switched calls.